While it's unlikely you'll ever run into malware for the Mac, you may want to consider an antivirus tool anyway—if not to protect yourself, but to protect your Windows-using friends from any malware you may inadvertently send their way. We think that Sophos Anti-Virus for Mac is the best choice, and it's free.

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Update: Our previous recommendation for Mac Antivirus was "nothing." We know many of you choose to use nothing, but we also know you don't come to the App Directory to not get any suggestions at all. Besides, malware is starting to become a bit more prevalent on the Mac, and even the safest browsing habits don't protect you completely. So, we now officially recommend Sophos as our Mac antivirus of choice.

Features

Compact, easy-to-use interface that can be used for custom on-demand scans of files, folders, and drives, or scheduled, periodic full scans of your Mac.

Also scans files on your Mac for known Windows malware, trojans, and viruses, and deletes or quarantines them so you don't risk spreading them to someone else via network share, USB drive, or email.

Deletes or quarantines known threats, gives you the option to quarantine anything suspicious that may be a new threat or dangerous file.

Runs quietly in the background, scanning emails, downloads, and any other files on access, stopping you from opening them before they can do any harm.

Light on system resources while running in the background.

Installs like any other Mac application, and uninstalls just as easily—no complicated packages or components to manage or configure.

Sophos' "Live Antivirus" feature updates your app the moment new threats are detected or found in the wild. The feature also performs real-time lookups to see if files accessed are in the SophosLabs database, even if they're unfamiliar to the app.

Supports OS X up to 10.8 and back to 10.4, and is completely free for all versions.

Where It Excels

Sophos actually has an excellent breakdown of the history of malware for the Mac going all the way back to 1982. The fact that the article exists should remind Mac users that while they're not the primary target for malware authors, they're by no means invulnerable. The size of the article however should issue some confidence that the risk—while present—is by no means critical.

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Sophos Anti-Virus for Mac stands out in a somewhat crowded field of Mac antivirus apps because it doesn't just scan your Mac's files and folders on demand, but it does it quietly in the background without tapping your already precious system resources in the process. The utility also keeps its own constantly-updating database of Windows viruses, trojans, and other threats, so if you inadvertently download a Windows virus or trojan that won't harm you, you don't run the risk of sending it off to someone else by forwarding the message, or you won't infect other computers on your network (or any Windows partitions or virtual machines you run on the same hardware) via shared drives. Sophos is smart enough to tell you "Hey, this won't hurt you, but we're going to quarantine/delete it so you don't accidentally email this attachment to someone else." That's a huge benefit—and it keeps you from being that guy no one likes.

Another banner feature Sophos offers that its competition doesn't is its live, real-time access to SophosLabs. "Live Antivirus," as it's called in the app, gives you an added layer of protection. The app automatically identifies and quarantines suspicious files, installers, and other packages that may not be well known threats yet, but definitely exhibit behavior suspicious enough that Sophos is looking into them.

Best of all though, in our testing, Sophos was one of the most resource-light antivirus apps on the Mac, which is impressive considering the features it offers.

Where It Falls Short

Sophos Anti-Virus for Mac isn't perfect, however. Even though it's pretty resource light, it wasn't the lightest in our tests. It just hit the sweet spot between resources and features. Also, support for Mountain Lion came a few months after its launch, so Sophos wasn't exactly right there with those people who upgraded on launch day.

The Competition

ClamXav 2 uses the open source ClamAV virus scanning engine. It can also detect both WIndows and OS X malware, scan on demand or on a regular schedule, and it's probably a bit more lightweight and easy on system resources than Sophos. It's compatible with OS X 10.5 or higher. The only trouble with ClamXav is that its definitions come a bit more sporadically than we'd like (daily, usually, sometimes, if they feel like it) and while performing scans is easy, tweaking all of the settings and getting the app scanning proactively is a little more effort than I'd like to see. Still, it's an excellent alternative, and one of the first you should check out if Sophos isn't cutting it for you.

Avira is another free utility worth a look. It also picked up 100% of the Mac malware that passed through it with no false positives. Avira's UI and options are perfect for non-technical users, and it offers strong protection against known Mac threats. Again, there's no Windows protection in the app, and both the scheduling and custom scan options are a bit anemic, so power users may want to shy away from it. If you're installing it on your non-technical friend or family-member's Mac however, it's a great option.

Whatever you do though, it's probably not necessary to plop down money for a Mac antivirus suite. The threats don't warrant you dropping money on software to keep you safe, and too many of them do more harm than good (especially the ones looking to get your money). They're just not worth it, especially when there are more effective, robust, free options like the ones above available to you.